Solvent drag across gramicidin channels demonstrated by microelectrodes

The competition of ion and water fluxes across gramicidin channels was asse ssed from the concentration distributions of both pore-impermeable and -per meable cations that were simultaneously measured by double-barreled microel ectrodes in the immediate vicinity of a planar bilayer. Because water movem ent across the membrane led to accumulation of solutes on one side of the m embrane and depletion on the other, the permeable cation was not only pushe d by water across the channel (true solvent drag); it also flowed along its concentration gradient (pseudo-solvent drag). For the demonstration of tru e solvent drag, a difference between the bulk concentrations on the hyperto nic and the hypotonic sides of the membrane was established. it was adjuste d to get equal cation concentrations at both membrane/water interfaces. Fro m the sodium and potassium fluxes measured along with membrane conductivity under these conditions, approximately five water molecules were found to b e transported simultaneously with one ion through the channel. in diphytano yl phosphatidylcholine membranes, a single-channel hydraulic permeability c oefficient of 1.6 x 10(-14) cm(3) s(-1) was obtained.